The present invention relates generally to nuclear medicine diagnostic apparatuses that use radiation. More particularly, the invention concerns a nuclear medicine diagnostic apparatus, positron emission computed tomography (PET) apparatus, and detector unit group suitable for performing multiple types of radiation examinations such as X-ray CT, PET, and single-photon emission computed tomography (SPECT).
The examination techniques that use radiation can be used to examine the insides of target bodies in a non-invasive fashion. Radiation examination techniques for human bodies, in particular, include, for example, X-ray CT, PET, and SPECT. All these techniques are intended to acquire images by measuring the physical quantities of a body to be examined, as integral data of radiation in the emission direction thereof, then back-projecting the integral data, and calculating the physical quantities of individual voxels within the target body. These techniques require processing vast volumes of data, and the rapid development of computer technology in recent years has enabled techniques to provide detailed images rapidly and at high resolution.
The PET and SPECT apparatuses used for diagnosis in nuclear medicine employ a technique that allows detection of the functional activity and metabolism at a molecular biological level that are not detectable with other diagnostic apparatuses such as an X-ray CT apparatus. The PET and SPECT apparatuses can provide functional images of target bodies, such as human bodies.
PET is a technique for acquiring images by administering to humans radioactive pharmaceuticals labeled with a positron emission nuclide such as 18F, 15O, or 11C, and measuring the distribution of the pharmaceutical. The types of pharmaceuticals used include, for example, 2-[F-18] fluoro-2-deoxy-D-glucose (18FDG), which is used to identify a tumor site by utilizing the characteristic that the pharmaceutical densely accumulates at the tumor tissue by means of glucose metabolism. The radionuclide that has been introduced into the human body decays and emits a positron (β+). Upon combining with an electron and decaying, the emitted positron itself emits one pair of annihilations (annihilation pair) having an energy of 511 keV. Since the two annihilations are irradiated in essentially opposite directions at 180±0.6 degrees, projection data can be obtained by simultaneously detecting the annihilations using multiple radiation detectors arranged around the target body, and storing radial data thereof. Back-projection of the projection data by using a filtered back-projection method, or the like, makes it possible to identify radiation positions, that is, the accumulating positions of the radionuclide.
SPECT is a technique for acquiring images by administering to target bodies, such as human bodies, a radioactive pharmaceutical labeled with a single-photon emission nuclide, and measuring the distribution of the pharmaceutical. A single γ-ray with an energy of about 100 kev is emitted from the pharmaceutical, and the energy of the emitted single γ-ray is measured using a radiation detector. The emission direction of the single γ-ray cannot be identified from its energy data measurements. For SPECT, therefore, a collimator is inserted in front of a radiation detector and the single γ-ray is detected only from a specific direction, whereby projection data can be obtained. As with PET, SPECT uses filtered back-projection or the like to obtain image data by back-projection of projection data. SPECT differs from PET in that because of energy measurement of the single γ-ray, simultaneous measurement is unnecessary, and hence in that the number of radiation detectors required is small. SPECT is also simple in apparatus configuration.
The above-described conventional PET, SPECT, and other nuclear medicine diagnostic apparatuses use a scintillator(s) as a radiation detector(s) to obtain an image. The scintillator requires converting an incident into visible light and then reconverting the light into an electrical signal via a photomultiplier. The scintillator has also had a problem in that since the number of photons generated during the conversion into visible light is small and since, as described above, two conversion process steps are necessary, energy resolution decreases and this does not always make diagnosing accuracy improvable. This decrease in energy resolution results in quantitative assessment being impossible, particularly during 3D imaging with PET. This is because the decrease in energy resolution necessitates the energy threshold level of the single γ-ray to be correspondingly reduced and because this results in detection of a large quantity of human body internal scattering which increases noise during 3D imaging.
In recent years, therefore, semiconductor radiation detectors are attracting attention as radiation detectors for use with nuclear medicine diagnostic apparatuses. The semiconductor radiation detectors convert an incident γ-ray directly into an electrical signal, and feature high energy resolution since a large number of electrons and hole pairs are generated.
The characteristics of these radiation detectors (scintillators and semiconductor radiation detectors) are typically temperature-dependent. The characteristics referred to here include time resolution and energy resolution. It is known that these characteristics improve during the use of the radiation detectors under low-temperature conditions.
Japanese Patent Laid-open No. 2005-128000 (Paragraphs 0058-0060) describes a cooling structure that efficiently cools signal processors and radiation detectors. Maintaining these radiation detectors at low temperature allows the improvement of both image quality and quantitative characteristics, and hence, more accurate diagnosis.